1. Field of the Invention
The present invention relates generally to optical pickup head apparatus and, more particularly, is directed to an optical pickup head apparatus for use, for example, in a recording and/or reproducing apparatus which records and/or reproduces a so-called optical disc such as a magneto-optical disc and the like.
2. Description of the Prior Art
An optical disc recording and/or reproducing apparatus is known for recording and/or reproducing a so-called optical disc such as a magneto-optical disc and the like. This type of optical disc recording and/or reproducing apparatus utilizes an optical pickup head apparatus of linear motor system to translate its objective lens. For example, U.S. Pat. Nos. 4,545,046 and 4,607,913 describe such type of optical pickup head apparatus. In that case, if two parallel guide rods are not supported very accurately, then this will impede a smooth movement of a movable portion to which an objective lens is attached. Further, in this type of optical pickup head apparatus, bearings attached to a movable portion become expensive. If the bearings are not equal in quality during the mass production, this will give rise to poor movement of the movable member.
Applicant's U.S. Pat. No. 4,864,170 describes a linear motor which can be applied to the aforenoted optical pickup head apparatus.
In order to gain a better understanding of the present invention, let us explain our previously-proposed linear motor (U.S. Pat. No. 4,864,170) with reference to FIGS. 1 and 2.
FIG. 1 is a perspective view of such linear motor, and FIG. 2 is a transverse sectional view taken on the line I--I in FIG. 1.
As shown in FIGS. 1 and 2, there are shown two permanent stator magnets 1 and 4 which are both magnetized in the thickness direction thereof. A base plate 2 is made of a magnetizable material, for example, iron and on which there are secured two outside stator yokes 3 and 5. The permanent stator magnet 1 is attached to the inner side of the outside stator yoke 3, and the permanent stator magnet 4 is similarly attached to the inner side of the outside stator yoke 5 with a predetermined clearance relative to the permanent stator magnet 1. Enclosing the permanent stator magnets 1 and 4, inside stator yokes 6 and 7 are secured to the base plate 2 with a predetermined clearance between the magnets 1 and 4 such that they may form a closed magnetic path together with the outside stator yokes 3 and 5.
There is shown a frame or bobbin 8, and at least the outer peripheral portion of the bobbin 8 is made of a nonmagnetic material. A pair of coils 9a and 9b are wound around two end portions of the bobbin 8, and through-holes 10 and 11 are formed through the bobbin 8 so that the through-holes 10 and 11 cross each other in the front and rear direction as well as the upper and lower direction.
The bobbin 8 has formed therethrough a pair of apertures 12 and 13 which penetrate the pair of coils 9a and 9b. The above-described inside yokes 6 and 7 are inserted into the pair of apertures 12 and 13.
Guide grooves 14, 15 and 16, 17 are formed on the inside surfaces of the apertures 12 and 13 and on the opposite surfaces of the inside yokes 6 and 7 in which the inside yokes 6 and 7 are not opposed to the magnets 1 and 4. As shown in FIG. 2, a pair of shafts 18 and 19 are attached to the insides of the guide grooves 14 and 15 formed on the apertures 12 and 13, and balls 21a and 21b are supported by supporting members 20a and 20b within these guide grooves 14, 15 and 16, 17. Therefore, the bobbin 8 is supported to the inside yokes 6 and 7 via the balls 21a and 21b, whereby the bobbin 8 can freely deviate along the longitudinal direction of the inside yokes 6 and 7.
A reflection mirror 22 for changing a light path is secured to a portion in which the through-holes 10 and 11 cross each other in the front and rear direction and in the upper and lower direction of the bobbin 8. An optical pickup 25 having an objective lens 24 is attached to the exit side of the vertically-elongated through-hole 11 via a mount 23. The optical pickup 25 is opposed to an optical disc D and includes a focusing motor 26 to move the objective lens 24 in the optical axis direction of the objective lens 24.
As shown in FIG. 1, a read-out optical system 27 is located in the direction in which the through-hole 10 formed through the bobbin 8 in the front and rear direction is extended in the rearward. The read-out optical system 27 comprises of a laser diode 28, a beam splitter 29, a collimator lens 30, a light receiving system 31, a photodiode 32 and the like. In accordance with the read-out optical system 27, a light beam emitted from the laser diode 28 travels through a light path formed of the collimator lens 30, the reflection mirror 22, the objective lens 24 of the optical pickup 25, the optical disc D, the objective lens 24, the reflection mirror 22, the collimator lens 30, the beam splitter 29 and the light receiving system 31, in that order and becomes incident on the photodiode 32.
In the previously-proposed optical pickup head apparatus constructed as described above, when the coils 9a and 9b provided at the two end portions of the bobbin 8 are both energized, the bobbin 8 is given a driving force according to a Fleming's left-hand rule provided on the basis of the action between the current and the magnetic field produced by the magnets 1, 4 and the yokes 3, 6 and 5, 7, i.e., a linear motor is constructed, whereby the optical pickup 25 is moved in the radial direction of the optical disc D. During this operation of the optical pickup 25, the objective lens 24 of the optical pickup 25 is driven by the focusing motor 26 so that the focusing of the objective lens 24 is adjusted.
In the prior-art optical pickup head apparatus shown in FIGS. 1 and 2, the linear motor for moving the optical pickup 25 is made compact in size because the bobbin 8 supporting the optical pickup 25 commonly utilizes the yokes 6 and 7 of the base plate 2 side as the guide shafts. However, the optical pickup 25 supported to the bobbin 8 is provided with the focusing motor 26 which moves the objective lens 24 and the focusing motor 26 and the feed linear motor constitute different magnetic circuits, whereby the focusing motor 26 and the feed linear motor are caused to interfere with each other between the two magnetic circuits. Thus, there is provided a magnetic neutral portion which frequently obstructs the movement of the optical pickup 25, i.e., the bobbin 8.
Further, the bobbin 8 is applied with the weights of the magnetic circuit of the focusing motor 26 so that the feeding mass is increased which results in that the access speed decreases. Simultaneously, the number of magnets forming the magnetic circuits is increased, which provides an increased cost.